The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => A7 & A10 Engine => Topic started by: bsa-bill on 11.07. 2011 19:41

Title: valve kissing piston
Post by: bsa-bill on 11.07. 2011 19:41
having a week where we are here there and everywhere for short times every day I decided it might be a good opportunity to fix the one and last oil seepage on the Rocket Gold Flash.
It's weeping out of the crankcase just under the camshaft trough, also a chance to swap the magnetos back onto thier respective bikes, not that it matters a lot.
Anyway it means engine out, but it's going to have to be done at some time so might as well be now when I have some time.
Got as far as removing the head and had a good gander and noticed the timingside inlet valve appears to be kissing the piston photo 1 the tweezers point to the mark, photo 2 shows the valve.
other piston is OK.
There is nothing on the piston to determine a front or back, the only mark on them is the "71" as in photo 3. both valves are in with the "71" facing the same way, (they read properly from the drive side
 the valve does not appear to be seated as well as it's neighbour, have not had time to take it out as yet.
Any advise as to best practise would be welcome
presuming the valve is fine is it ok to take a bit off the piston valve pocket, would this require doing the same to other piston
other thoughts would be to grind it in a bit more or try a new valve see if it's bent

As always your thoughts appreciated
Title: Re: valve kissing piston
Post by: MG on 11.07. 2011 20:11
Hi Bill!
That is bad news indeed.
On the second picture, one pocket seems to be a bit bigger than the other one if I'm not mistaken (hard to tell)? If so, then the piston is the wrong way 'round, the ex valves are smaller than the inlet valves (always, except on some pre-war Bugattis and the botched A7 head sold on ebay recently  *whistle*), hence the wider pocket (if there is any) goes to the inlet side (rear of the engine).
Title: Re: valve kissing piston
Post by: chaterlea25 on 11.07. 2011 20:13
Hi Bill,
Am I correct in thinking that you had problems with sticking valves on this engiine early on???
The valve will be bent from contacting the piston, they hammer straighter after a few revolutions, but they are still out of true, the  seat will have suffered somewhat as well.
I have just but back together a ZB goldie that this has happened to,
The sticky valve was due to tank sealer melting and condensing back onto the inlet tract and valve

Another possibility is that a pushrod was not in place properly causing the valve to be open too far or if the pushrod was not properly in the follower it opened at the wrong time

buggersh*tanf*ck *ex* *ex*

John O R
Title: Re: valve kissing piston
Post by: MG on 11.07. 2011 20:25
Good thinking, John. A sticky valve is a very likely cause.

A good way to check is filling the valve pockets with plasticine, putting the head and rockerbox back on, and turning the engine over a few revolutions. Then take the head back off and you will see the imprints of the valves in the plasticine. The thickness of it will show you how much clearance you have between valve head and piston crown. It is a tedious task, but will show you what's going on and give you peace of mind.
Title: Re: valve kissing piston
Post by: bsa-bill on 11.07. 2011 20:53
I did have a sticking valve John but it was the ex valve on the same cylinder, that's not to say that the inlet one has not been sticky at some time.
MG I had the same thought when I had the head off last but measured them and seemed to be the same, also I could have sworn the pocket were offset one in relation to the other, then convinced myself it was an optical illusion, will measure carefully tomorrow also order two new valves just in case, I replaced the ex ones with ones from C&D autos, could be the ones I got originally are poor quality, can't recall where I got them now, this restoration is taking so long some of the NEW parts I bought are now classic.
Any way if the valve pockets measure different I'll turn the piston around (barrels have to come off anyway).
Might as well make the best of the situation and change back to std gearing, I was after some extra acceleration at the expense off top speed but the thing goes that well (even with piston valve handshaking) that std gearing will be fine and might give a bit extra speed to the maggie when kicking over.

Will have a good measure tomorrow and report on findings
Title: Re: valve kissing piston
Post by: trevinoz on 11.07. 2011 22:31
Have you had new seats fitted Bill?
This can happen if the new seat stands a little proud.

Title: Re: valve kissing piston
Post by: bsa-bill on 11.07. 2011 22:43
Hi Trevor- I bought the head from Roy Shearwood, it had been refurbished with new guides, unleaded seats and vapour blasted, so you could well have a point there.
Have just had another little peak down the shed, both valve pockets in the piston are the same size.
think I'll order new valves and use the old one to grind in a tad further, if this action does not upset the seat profile, also will have a good look at the pistons I had in originally to see if either of them shows the same mark, if they are ok then that would point to the valve.
Is that logical?
Title: Re: valve kissing piston
Post by: muskrat on 11.07. 2011 23:20
G'day Bill,
             from the pic the inlet valve looks rather large or bent back sitting up on the seat, and that matches the mark on the piston.
If the EX valve was sticking open long enough the IN valve could have clipped it and bent.
Title: Re: valve kissing piston
Post by: trevinoz on 11.07. 2011 23:29
         If you can, compare your seats to an original head.
If yours are too high, the best solution is to have them recut, in my opinion. Too much work trying to grind them down with an old valve and paste plus a machine cut will be a better job. [Hopefully]

Title: Re: valve kissing piston
Post by: Topdad on 12.07. 2011 13:49
Hi Bill my one and only problem with the rebuild was a sticking valve , kissed the piston and needed a long push home plus a replacement. Once you've got that valve out you should get a picture clearer 'cause it looks proud to my old eye's . If the valve isn't bent I'd try a recut as Trev suggested ,you'd be in the shed til christmas trying to grind it in. Best wishes Bob.
Title: Re: valve kissing piston
Post by: RichardL on 12.07. 2011 14:26
Without disagreeing with the sage advice given,  I have a question.  The valve looks to be large diameter.  The pistons are moderate compression (7.5 or 8:1?).  Is it possible that these pistons are not notched for large D.  valves,  considering that the kiss is on the lip?

Richard L.
Title: Re: valve kissing piston
Post by: bsa-bill on 12.07. 2011 17:42
Bit of an update, been out most of the day so not much progress.

Richard L - only one valve is involved, the other three are OK
Trev - I don't have another (alloy) head to compare and with a bit more thought and inspection the valve is just touching the edge of the pocket, if the seat was the problem then the valve I think would still go in the pocket, this valve is missing the pocket by a whisker.
Also the original pistons (9.5) - one of them does have a very slight mark on the edge of the pocket, hardly discernible but very possible from the same problem, these pistons only did about fifty miles and had started to pick-up (re bored to tight).
One common comment from most of you concerned the valve looked large, now when I started this project I had a big valve head to go on it, I wonder if I ordered big valves for it, only thous in it as I understand, but it's only a thou or two that the things catching by.
The valve when rotated in the head does not show any variance, however I have two new valves ordered from C&D, should arrive tomorrow.
AFAIK an A10 piston is an A10 piston and the higher compression ones would normally only be used on Rockets and so forth so you would presume the pockets would so sized, you never know though.
One thing I know is I will never run a project like this again, buying stuff as and when it comes up at a good price to use in the future can backfire, no I reckon buy it as you need it then you will know better what you need - you live and learn.
Title: Re: valve kissing piston
Post by: chaterlea25 on 12.07. 2011 22:47
Hi Bill,
Can you give us a measuremant of the valve head, I have some out at the moment and can compare
If the offending valve has a bigger head than the others then the length sticking up for the valve sping will be shorter
If the head is the correct dia. then the seat must be proud somewhat??

John O R
Title: Re: valve kissing piston
Post by: bsa-bill on 13.07. 2011 15:24
Hi John
new valves from C&D arrived this morning, I'm just back from other duties but have taken the new ones down to the shed for comparison.
Yep the originals are to big, I must have ordered big head valves early on and forgotten they were bigger.
They are in fact 43/44 thou bigger than the new ones which are road rocket, lucky I got away with it I think, and just one valve kissing, must be a bit less tolerance on that pocket.
The new  ex valves I got some months back to replace a sticking one are the same size as the original big valve head ones, (also the same size as spare new Flash ones, must look up big valve head specs).

Think it's time for paper and pencil diary of what I buy and do.
To paraphrase an old saying "Iv'e forgotten more than I'll ever know"

Thanks for the heads up folks, it was the "looks large" that got me on the right track